1. Field of the Invention
The invention relates generally to a control apparatus and control method for an internal combustion engine. More specifically, the invention relates to a control apparatus and control method that controls an internal combustion engine where the operation for controlling the amount of air taken in a cylinder is switched between a throttle operation and a non-throttle operation.
2. Description of the Related Art
An exhaust gas control catalyst for an internal combustion engine cannot satisfactorily purify exhaust gas, if the temperature thereof is below a predetermined activation temperature. Accordingly, if an internal combustion engine is started while it is cold, the temperature of the catalyst needs to be increased to the activation temperature as quickly as possible.
Japanese Patent Application Publication No. JP-A-06-213056 describes a system that performs a catalyst warming control to retard the ignition timing, which increases the temperature of the exhaust gas, after an internal combustion engine is started. The system stops the catalyst warming control, if it is determined, during the catalyst warming control, that a vehicle will start running. Thus, rough operational feeling that a driver may feel as the vehicle starts running is prevented.
In a spark ignition internal combustion engine, the amount of air taken in a cylinder is usually controlled by regulating the opening amount of a throttle valve. However, in recent years, internal combustion engines with variable valve systems that change the valve-open duration and the valve lift amount of intake valves have been proposed. In such internal combustion engines, the amount of air taken in a cylinder is controlled mainly by operating the variable valve system, so-called non-throttle operation. Controlling the amount of air taken in the cylinder by the non-throttle operation reduces pumping loss considerably, and significantly improves fuel efficiency.
However, because the pumping loss is small and the engine load is low during the non-throttle operation, the amount of exhaust gas is also small. Accordingly, it takes a long time to increase the temperature of the catalyst to the activation temperature. As a result, the amount of exhaust emissions is likely to increase.